Evaluation of Chemical Extractants for Predicting Lowland Rice Response to Zinc in Highly Calcareous Soils

Abstract

Zinc (Zn) deficiency has been observed in lowland rice (Oryza sativa L.) in Iran. Relatively high pH and abundant calcium carbonate of soils are responsible for Zn deficiency in the plant. However, the Zn status in paddy soils of Iran has not been reported. Therefore, available Zn in 20 highly calcareous soils with a wide range of chemical properties was evaluated with DTPA-CaCl₂, EDTA-(NH₄)₂CO₃, DTPA-NH₄HCO₃, EDTA, and Mehlich 3, using rice as a test crop. The results showed that extraction of Zn decreased in the order: Mehlich 3, EDTA, DTPA-NH₄HCO₃, EDTA-(NH₄)₂CO₃, DTPA-CaCl₂. A significant positive correlation was found between Zn extracted by all chemical extractants with the exception of EDTA. The highest correlation was observed between DTPA-CaCl₂ and EDTA-(NH₄)₂CO₃ extractable Zn. Application of Zn significantly increased mean top dry weight, Zn concentration, and Zn uptake by rice. Apparent Zn recovery of applied Zn was quite low in all soils, probably due to conversion of Zn to unavailable forms. The critical level of soil available Zn with DTPA-CaCl₂, EDTA-(NH₄)₂CO₃, DTPA-NH₄HCO₃, EDTA, and Mehlich 3 was 0.30, 0.70, 0.76, 2.2 and 2.5 mg kg⁻¹, respectively. Plant dry matter and Zn uptake were significantly correlated with DTPA-CaCl₂, EDTA-(NH₄)₂, DTPA-NH₄HCO₃ and Mehlich 3 extractable Zn. However, Zn extracted by EDTA-(NH₄)₂CO₃ and DTPA-CaCl₂ best explained the variation in plant dry matter, whereas DTPA-NH₄HCO₃ extractable Zn was more appropriate to predict the variation in Zn uptake by rice. Moreover, inclusion of pH and ECe in a multiple regression equation led to the best prediction of top dry weight and Zn uptake by the EDTA-(NH₄)₂CO₃ soil test. It seems that EDTA-(NH₄)₂CO₃ is the most suitable chemical extractant to differentiate the Zn responsive paddy soils from nonresponsive ones in view of its high statistical relationship with the rice growth parameters.

Acknowledgments

We wish to thank the Shiraz University Research Council for approval and providing financial support for Research Project No. 74-AG-903-525.